1. Field of Invention
The present invention relates to a planarization process using borophosphosilicate glass. More particularly, the present invention relates to a method for preventing the formation of recesses in the borophosphosilicate glass (BPSG) surface caused by rinsing during wafer-cleaning operation. Without the recesses, the possibility of metal bridges linking different metal interconnects and leading to a short-circuiting of the interconnect lines in subsequent metal laying operation will be avoided.
2. Description of Related Art
Borophosphosilicate glass (BPSG) has a rather low glass transition temperature and therefore can easily flow in a thermal operation. Moreover, boropliosphosilicate glass has a much lower residual mechanical stress than an ordinary silicon dioxide layer because of the boron and phosphorus present. Consequently, borophosphosilicate glass is now extensively used as the first dielectric material for laying over a substrate after MOS devices are formed in the wafer. Especially, the BPSG serves as inter dielectric medium for carrying out the planarization procedures before the metallization operation.
In general, the transition temperature of borophosphosilicate glass depends on the amount of boron and phosphorus present, and will normally decrease with an increasing amount of those elements. However, when the amount of boron and phosphorus inside the borophosphosilicate glass is too high, the borophosphosilicate glass itself is not too stable. Subsequently, in the process of cleaning using a RCA solution, the phosphorus inside the BPSG will react with the constituents of the solution and form recesses on the surface of the originally planar BPSG layer. Therefore, the amount of boron and phosphorus inside the BPSG must be controlled within definite limits.
The conventional method to reduce the recesses in the BPSG layer due to cleaning is to monitor the concentration of the doped substances using a control wafer and generate an optimized concentration. When the concentration level of phosphorus inside BPSG actually changes, the change can only be discovered later. In other words, the conventional method can only give the actual concentration level of added phosphorus inside BPSG, but could not make any immediate response to restore back to a fixed concentration level.
FIGS. 1A through 1D are cross-sectional views showing a series of preparatory steps necessary prior to metallization including planarizing borophosphosilicate glass layer, etching contact opening and final cleaning operation according to a conventional method. First, as shown in FIG. 1A, a substrate 10 having a MOS structure already formed thereon is provided. The MOS structure includes a field oxide layer 11, a gate 12 and source/drain terminals 13 and 14. Then, a borophosphosilicate glass layer 15 is formed above the substrate 10. This is followed by a planarization process. In the subsequent step, as shown in FIG. 1B, a photoresist layer is formed over the borophosphosilicate glass layer 15 and then patterned to form a mask 16. Using the mask 16 as a protective layer, those portions of the borophosphosilicate glass layer 15 not covered by the mask are removed forming contact windows 17. After the removal of the photoresist mask layer 16, a structure as shown in FIG. 1C is obtained. Next, the substrate 10 is cleaned using a RCA solution. Because the phosphorus in a BPSG layer can easily react with the constituents in the RCA solution, the phosphorus on the surface of a BPSG layer can be removed during cleaning operation. If the BPSG layer 15 has an inappropriate level of phosphorus concentration, phosphorus on the surface of the BPSG layer 15 can be leached away during the cleaning operation to form recesses 18 as shown in FIG. 1D. Subsequently, when metal is deposited into the contact windows 17, the recesses 18 will also be filled and the metal there is difficult to remove. These metal-filled recesses now act as `bridges` connecting different metal lines leading to short-circuit. FIG. 1E is a top view showing the surface of a borophosphosilicate glass layer after a metal laying operation. After cleaning with a RCA solution, recesses 18 are formed on the borophosphosilicate glass 15 surface. When the recesses adjacent to the contact windows 17 are linked together through a metal deposition, bridging will occur resulting in short-circuit between two supposedly unconnected metal conducting wires.
In light of the foregoing, there is a need in the art to provide a method for preventing the formation of recesses in the borophosphosilicate glass layer.